1. Technical Field
The present invention relates to a cordless power tool.
2. Background Art
Heretofore, there has been known a cordless power tool (hereinafter referred to as a power tool) which operates upon reception of power supply from a battery pack attached detachably to a main body, such as a jig saw, a stud cutter, or the like. In order to operate such the power tool, large electric power is required. Therefore, in addition to a Ni—Cd battery and a nickel-hydride battery, recently, a lithium-ion battery is widely used as a rechargeable battery of a battery pack.
There is an advantage that the lithium-ion battery can supply large electric power and realize size/weight reduction, while it is necessary to strictly cope with ignition or the life decrease of the battery due to overcharge, overcurrent and overdischarge. For example, regarding the lithium-ion battery, it is thought: in case that the overdischarge occurs or the over current flows in a load, the temperature of the battery increases thereby to lower the life of the battery, or a motor for a power tool is broken; and in case that the overcharge is performed, ignition occurs. For the above problems, there has been known a battery pack having a protection unit (refer to, for example, Patent Document 1) which shuts off supply of electric power to a power tool in case that overcurrent and overdischarge have been detected. In the Patent Document 1, an FET (field effect transistor) is used as the protection unit. Further, such the constitution that the FET as the protection unit is provided not for the battery pack but on the body side of the power tool is also proposed.
[Patent Document 1] Japanese Patent No. 3222951
However, when the power tool is operated with high load, the protection unit may generate heat and causes an erroneous operation due to an influence of the heat. Further, there is fear that the life of the protection unit lowers.